Method for storage tank leak detection having ground water compensation

ABSTRACT

A method is described for enabling leak detection of an underground storage tank surrounded by a water table. The method begins by sealing a test probe in the tank, the test probe having an elongated substantially hollow core with first and second ends, a housing for receiving the second end of the hollow tube and being vented to the fluid product, a low temperature coefficient medium supported in the substantially hollow core of the elongated tube, and a liquid seal supported in the housing between the medium and the fluid product. The method continues by then establishing and maintaining a pressure equilibrium between the hollow core of the test probe and the head space. An inert gas such as nitrogen is then introduced into the head space to overpressurize the head space by an amount sufficient to reestablish a leak at any break in the storage tank despite presence of the water table surrounding the storage tank. Once the tank contents are stabilized, the precision leak test is performed.

This is a continuation-in-part of prior copending application Ser. No.07/312,864, filed Feb. 21, 1989, now U.S. Pat. No. 4,914,943.

TECHNICAL FIELD

The present invention relates generally to underground storage tank leakdetection systems and methods and more particularly to techniques forcompensating for the presence of ground water in connection with suchleak detection.

BACKGROUND OF THE INVENTION

Underground storage tanks are used to store hazardous substances andpetroleum products. It is estimated that a significant proportion of thenearly five million tanks in the United States are leaking harmfulproducts into the environment. To ameliorate this problem, theEnvironmental Protection Agency (the "EPA") has recently promulgatedregulations which require that any leakage exceeding a rate of 0.05gallons per hour be detected and contained.

Methods for detecting leaks in underground storage tanks are well knownin the prior art. Most of these techniques use a quantitative approachto identify a leak or to determine leak rate based on a measurement ofvolumetric changes of the stored product in the tank. The capability ofprior art leak detection methods to accurately measure leakage isaffected by certain variables such as temperature change, tankdeformation, product evaporation, tank geometry and the characteristicsof the stored product. In addition to these variables, the presence ofground water around the tank may completely mask an actual leak or atleast slow the rate at which the stored product is leaking.

In particular, the water table of the soil in which the tank is buriedcan vary in height depending of a number of factors including but notlimited to location, time of year and amount of rainfall. If the watertable is above the location of a hole or break in the tank, the groundwater exerts a pressure on that break which counteracts the pressureexerted by the product in the tank. When the water level is above theproduct level, the pressure exerted by the ground water is greater thanthe pressure exerted by the product against the break, and thus theground water will flow into the tank. If the product level is above thewater table level, in some cases the pressure exerted by the productwill be exactly balanced by the pressure of the ground water at thebreak and thus leakage out of the tank will be prevented or greatlyreduced. In either case, the true nature and scope of the leak cannot beaccurately detected.

One way of compensating for ground water "masking" is to simply postponethe leak test until such time as the ground water is below the bottom ofthe tank. This approach is, of course, highly impractical and expensive.Another approach is to conduct two consecutive tests, each at adifferent fluid level in the tank, and compare leak rates. The leakrates will differ if there is a leak due to the differing headpressures. This approach theoretically is independent of ground waterlevels because the difference in leak rates should be measurable whetherground water is present or not. In practice, the conducting of twoseparate leak tests is costly and time-consuming. The technique is alsounreliable because the differences in leak rate from changes in headpressure are often obscured by temperature induced volumetric changes.

Accordingly, there is a need for a reliable and economical method foreliminating ground water "masking" effects in a storage tank leakdetection system which overcome these and other problems associated withprior art techniques.

BRIEF SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for compensating"masking" effects induced by ground water during storage tank leakdetection.

It is another object of the present invention to provide a leakdetection apparatus that utilizes an isobaric emulated hydrostatic headprocedure for accounting for the effects of ground water surrounding anunderground storage tank.

It is a further object of the invention to describe precision leaktesting methods wherein the effect of ground water is detected andsubstantially eliminated during the tank testing procedure.

It is still another object of the invention to decrease the timerequired to perform a precision leak test for an underground storagetank while simultaneously reducing the cost and complexity of suchtesting.

These and other objects are provided by the novel method and of thepresent invention wherein a a test probe is maintained in pressureequilibrium with a head space of the tank while the space is pressurizedby a predetermined amount to establish a pressure disequilibrium betweenthe tank contents and the water table. This procedure provides effectivecompensation of any ground water masking effects.

The foregoing has outlined some of the more pertinent objects of thepresent invention. These objects should be construed to be merelyillustrative of some of the more prominent features and applications ofthe invention. Many other beneficial results can be attained by applyingthe disclosed invention in a different manner of modifying the inventionas will be described. Accordingly, other objects and a fullerunderstanding of the invention may be had by referring to the followingDetailed Description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference should be made to the following DetailedDescription taken in connection with the accompanying drawings in which:

FIG. 1 is a sectional view of an underground storage tank having a fluidproduct stored therein and surrounded by a high water table, as well asa sectional view of a testing probe for use in providing leak detectionin connection therewith; and

FIG. 2 is a detailed sectional view of the testing probe shown in FIG.1.

DETAILED DESCRIPTION

With reference now to the drawings wherein like reference charactersdesignate like or similar parts through the several views, FIG. 1 is aperspective view of an underground storage tank 10 in which an apparatusfor leak detection is used. As used herein, the term "underground"refers to any storage tank with at least some portion of its volumeburied below ground. Such tanks are commonly used, for example, to storehazardous substances and hydrocarbon products such as gasoline and crudeoil.

The underground storage tank 10 has a base 12 and is mounted with itslongitudinal axis horizontal. The tank is generally located several feetbelow a manhole access port 14. A vertical riser or fill pipe 16 isprovided to connect an upper end of the tank 10 to the manhole accessport 14, and a standpipe (not shown) is used to fill the tank. Inparticular, the tank 10 supports a fluid product 18, e.g., hydrocarbonfuel, which has a predetermined volumetric coefficient of expansion perdegree Fahrenheit or Centigrade (a "temperature coefficient"). The tankincludes a head space 15 above the level of the fluid product 18.

The level or height 20 (and therefore the volume) of the product 18 isaffected by product leakage from the tank, designated by arrow 19, orleakage of foreign products into the tank, designated by arrow 21. If ahigh water table (or perched transient water) surrounds the tank, suchwater can mask the actual leakage of the product out of the tank and insome cases actual prevent such leakage until the water table drops. Thusaccurate and reliable leak testing must account for the existence ofhigh ground water surrounding the storage tank. If the water table 17 isabove the location of a hole or break 23 in the tank, the ground waterexerts a pressure on that break which counteracts the pressure exertedby the product in the tank. When the water level is above the productlevel, the pressure exerted by the ground water is greater than thepressure exerted by the product against the break, and thus the groundwater will flow into the tank.

Referring back to FIG. 1, a test probe 200 is provided for use in thestorage tank leak detection procedure. In the preferred embodiment, thistest probe is of the type generally described in copending applicationSer. No. 07/312,864, filed Feb. 21, 1989, and more specifically asdescribed in copending application Ser. No. 07/480,809, filed Feb. 16,1990. Both of these applications are incorporated herein by reference.

Referring briefly to FIG. 2, a detailed view is shown of a preferredembodiment of the test probe 200. The test probe 200 includes a tube202, preferably formed of a low temperature coefficient material such asgraphite or the like, which is removably secured to the housing 25 by acoupling assembly 206. As described earlier, housing 25 includes a base27 and a circular sidewall 29, and is vented to the hydrocarbon product18 through opening 31 (or a valve) in the top portion of the housing 25.

Coupling assembly 206 includes a cap 208 having a first end 210, asecond end 211 and a threaded sleeve portion 212. The first end 210 ofthe cap 208 supports a tube 214, with the tube 214 corresponding to thesecond end 28 of the pressure tube 24 as described with respect to FIG.5. Coupling assembly 206 also includes a coupling nut 216 having anannular ring portion 218 and a sleeve 220. Sleeve 220 includes athreaded portion 222 which is adapted to be threaded to the threadedportion 212 of the cap 208. The coupling assembly 206 also includes aflange 224 having a sleeve bonded or otherwise secured to an interiorwall 226 of the outer tube 202. The flange 224 includes an annular ringportion 228 adapted to mate with the annular ring portion 218 of thecoupling nut 216. Cap 208 also includes an aperture 219 for supporting atemperature sensor (not shown).

The housing 25 supports a non-reactive, liquid barrier seal between thehydrocarbon product 18 and a low temperature coefficient mediumsupported in the remainder of the test probe 200. The liquid barrierseal 33 comprises a working fluid that is immiscible to both the product18 and the low temperature coefficient medium. The seal thereforeprevents the hydrocarbon product 18 from mixing with the low temperaturecoefficient medium and vice versa. In the preferred embodiment, theworking fluid 33 is a flourinated silicone and the low temperaturecoefficient medium 35 is then preferably deionized or distilled water.

Referring simultaneously to FIGS. 1-2, the distilled water or other lowtemperature coefficient medium 35 supports a float 50 which is used byan interferometer 52 to sense variations in the level of the medium. Thefloat 50 is located within a inner tube 230 which is in turn locatedwithin an outer tube 232. The tubes are thus separated by a space 231.Tubes 230 and 232 are preferably formed of stainless steel, aluminum orcopper, and are supported within the graphite tube 202 by a pair of tubeflanges 234 and 236. In particular, the graphite tube 202 has an openingin its upper end portion adapted to receive the upper tube flange 234.The upper tube flange 234, which includes a vent channel 235, is securedwithin the opening of the graphite tube 202 by a laser support flange238 having an annular ring portion 240 and a sleeve 242 bonded orotherwise secured to the outer wall 244 of the graphite tube 202. Thelower tube flange 236 has a threaded sleeve portion 245 that isthreadably secured to a t-shaped coupling 246 having threads 248.Coupling 246 also includes an opening 250 for supporting a temperaturesensor (not shown).

Upper and lower tube flanges 234 and 236 include appropriate radialsupport surfaces 234a and 236a for supporting the outer tube 232. One ormore spacer sleeves 252 can be provided between the outer tube 232 andthe inner wall of the graphite tube to stabilize the outer tube. Thelower tube flange 236 includes an inner sleeve 236b for receiving abottom end 254 of the inner tube 230. An upper end 256 of the inner tube230 is in turn bonded or otherwise secured to a first end 258 of astabilizer tube 260. Tube 260 has a sleeve 262 about which a bellows 264is provided. Bellows 264, which allows longitudinal expansion andcontraction of the inner tube 230 as will be described, is secured to afacing portion 266 of the upper tube flange 234.

The remainder of the test probe 200 comprises a conduit 268 having upperand lower ends 270 and 272. The lower end 272 is threadably secured toan aperture 273 centrally-located in the cap 208. The upper end 270 isin turn attached to a quick disconnect assembly 274 by a coupling nut276. The quick disconnect assembly is conventional and includes upperand lower sections 276 and 278. Therefore, the housing 25 and tube 214are separable from the remainder of the probe 200 by unscrewing thecoupling nut 216 and separating the first and second sections 276 and278 of the quick disconnect assembly 274.

A column of the low temperature coefficient medium 35 is thereforesupported (above the barrier 33) in the tube 214, the conduit 268, thecoupling nut 276, the quick disconnect assembly 274, the coupling 246,the lower tube flange 236 and the inner tube 230.

The tube 200 is formed of graphite or some other similar low temperaturecoefficient of expansion material. The graphite tube insures thatlongitudinal temperature variations in the fluid product along thelength of the tube 202 are not transmitted to the medium 35 that issupported in the inner tube 230. Additional isolation of the medium 35from such temperature variations is further provided according to theinvention by thermally isolating the tube 202 from the inner tube 230 inwhich the low temperature coefficient material is supported. In thepreferred embodiment, this thermal isolation is provided by outer tube232 and by evacuating the space 231 located between the inner and outertubes 230 and 232. This vacuum is preferably created during manufactureof the test probe. Alternatively, a low temperature coefficient fluidcan be introduced into the space 231 by a pump 233 vented through theupper tube flange. The pump circulates the fluid to maintain suchadditional thermal isolation. As also described above, the bellows 264is further provided surrounding an upper portion of the inner tube forallowing longitudinal expansion and contraction of the inner tube 230.Thus even if the thermal isolation provided by the outer tube and theevacuated space 231 (or circulating fluid) does not completely eliminateall temperature variations along the inner tube length, the bellows 264provides additional compensation if needed.

Referring back to FIG. 1, the method of the present invention can now bedescribed in detail. According to the method, a pressure seal 41 isfirst supported adjacent the top portion of the fill pipe 16. Preferablythe pressure seal is made of rubber or the like; alternatively, the sealis an inflatable cuff. The pressure seal includes a first aperture 43afor receiving the probe 200, a second aperture 43b for receiving a probevent line 45, and a third aperture 43c for receiving a gas line 47 forthe purposes to be described. After placement of the pressure seal(and/or inflation of the inflatable cuff), all other tank vent and/ormanifold lines including the standpipe are sealed.

The test probe is then vented to the head space 15 by connecting theprobe vent line 45 through the second aperture 43b to the interior ofthe tank. The connection of the probe vent line 45 to the head spaceinsures that the low temperature coefficient medium 35 supported in theinner tube 230 of the probe is at the same pressure as the head space15. Of course, the probe can be vented to the head space prior tosealing off the remainder of the tank vent lines by connecting the probevent line 45 prior to inflating the inflatable cuff. Once sealing hasbeen effected, the probe and the head space remain in isobaric (i.e.,atmospheric) pressure equilibrium. If desired, the test probe and thepressure seal can be placed into the fill pipe at the same time.

To provide effective ground water compensation, the method of thepresent invention includes the step of then pressurizing the head space15 through the introduction of a pressurized gas through line 47.Preferably, an inert gas, such as nitrogen, is supplied from gas source51 via regulator 53 through the gas line 47 and into the head space 15for this purpose. The gas could also be helium, oxygen, hydrogen orother inert gas that does not mix with the product stored in the tank.Thus, according to the method, the test probe is maintained in apressure equilibrium with the head space while the head space isessentially overpressurized. This operation creates a pressuredisequilibrium between the tank contents and the surrounding watertable, thus reestablishing the outflow of the product at the break inthe tank. Once the tank contents stabilize and the leak isreestablished, accurate leak detection is effected using the test probe.

According to the method of the present invention, the test probe ismaintained in pressure equilibrium with the head space of tank while thespace is pressurized by a predetermined amount. In the preferredembodiment, dry nitrogen is introduced into the head space with the lowpressure regulator and allowed to stabilize at a pressure sufficient toproduce a hydrostatic head required to reintroduce a nominal 0.1 gallonper hour (gph) leak. The degree of hydrostatic pressure applied ispreferably calculated by the following formula:

    Overpressure=[(Fluid Height@95%)/232 in3] * Density,

where the Fluid Height is the height in inches of the medium in theinner tube 230 and Density is the density of the product 18 measured inpounds per gallon. Thus, for a tank with 6 pounds per gallon densityfuel and a 95% full column height of 85 inches, the formula returns amaximum tank bottom head pressure of 2.2 pounds per square inch.Consequently, the tank head space would be overpressurized to 2.2 psiwith nitrogen, which overpressure would then also be seen by the innertube of the probe. After the tank is stabilized to accommodate theinstantaneous tank end wall deflection induced by the overpressure, theprecision leak test is carried out. Since overpressurizing the headspace establishes a pressure disequilibrium between the tank contentsand the water table, the leak becomes active (and thus detectable)despite the presence of ground water surrounding the tank.

The method of the present invention is also applicable in volumetrictank testing methods. One such method involves overfilling the tank intothe delivery fill pipe which serves to magnify the volume/height changesof a fluid leak. Thermal compensation is performed by vertical samplingof the temperature of the product in the tank and correcting themeasured height change for thermal expansion or contraction of thefluid. By pressurizing the delivery fill pipe and the height measuringapparatus as described by the teachings of this invention, a leak may bereestablished in the presence of ground water around the tank.

In another volumetric method, a reservoir and pump arrangement is usedto maintain constant hydrostatic pressure in the fill pipe by adding orwithdrawing fluid to maintain a constant level. The nettemperature-corrected volume of added or subtracted fluid is thenconverted into a net leak/ingress rate. By pressurizing the fill pipeand reservoir assembly according to the teachings of the presentinvention, the method may be used when the tank breach is surrounded byground water.

It should be appreciated by those skilled in the art that the specificembodiments disclosed above may be readily utilized as a basis formodifying or designed other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

What is claimed is:
 1. A method for leak detection of an undergroundstorage tank surrounded by a water table, the tank for supporting afluid product and including a head space between the fluid product and atop portion thereof, comprising the steps of:(a) sealing a test probe inthe tank, the test probe having an elongated substantially hollow corewith first and second ends, a housing for receiving the second end ofthe hollow tube and being vented to the fluid product, a low temperaturecoefficient medium supported in the substantially hollow core of theelongated tube, and a liquid seal supported in the housing between themedium and the fluid product; (b) maintaining the hollow core of thetest probe and the head space in pressure equilibrium; and (c)introducing a gas into the head space to overpressurize the head spaceby an amount sufficient to reestablish a leak at any break in thestorage tank despite presence of the water table surrounding the storagetank.
 2. The method as described in claim 1 further including the stepsof:(d) allowing the fluid product in the tank to stabilize; and (e)performing a precision leak test using the test probe.
 3. The method asdescribed in claim 1 further including the step of sealing any tank ventlines prior to introducing the gas into the head space.
 4. The method asdescribed in claim 1 wherein the gas is nitrogen which is introducedinto the head space via a low pressure regulator.
 5. The method asdescribed in claim 1 wherein the gas is inert.